The Fire Brave feat grants ogres, among other things, the ability to "ignore size or species penalties to disguise checks". So a fire brave ogre can disguise him or herself as a member of any other species (but not gender...) with no more difficulty than disguising himself as some other fire brave ogre.
How does this work, exactly? I can see three ways:
1) Altered perception. Our Fire Brave remains twice as tall as a human, with horns and all the rest, but people just perceive him/her as something else. However, certain identifying features remain - eyes, voice, quirks etc. - that still need skill to conceal.
2) Actual physical alteration. Fire Braves can retract their horns, compress their body mass, and alter their skin and hair colour etc. without losing any of their standard physical power and abilities. Since they're still very strong and heavy, they're good at all the things that large people are good at... and also somehow still just as bad at sneaking (loud footfalls?), easy to hit (not quite as well-coordinated?) and still taking up just as large a base size (...) as they always did.
3) It just does.
Which of these is better, do you think? EDIT: And can you literally disguise yourself as any other species you like? (A small dog, a dragon, a cow...)